a year or age wim mat contained in me Dirtn certificates ot me same chii-dren. In the case of children born in 1949, they were able to do the same for deaths in the second year. Thus they could examine the association between infant mortality and a number of variables, including the particularly pertinent ones here of maternal age and parity and of social class of the family. Since they were dealing with a huge population-1,322,150 single, legitimate live births—associations that might never have been apparent in smaller groups stand out clearly. The wealth of detailed findings is published in a series of eight papers under the general title, "Social and Biological Factors in Infant Mortality" (20-27).* Figure 1 shows their data on variations with mother's parity of post-neonatal mortality (deaths between 1 month and 1 year of age) in different social classes (20). Social class differences in mortality rates are clear, as Social classes 20- IV & V 1234 £5 1234 £5 1234 Mother's parity (number of children) Figure 1. Variations with parity in postneonatal (1 month to 1 year) mortality from infections and congenital malformations in different social classes, England and Wales, 1949-1950, using social class scale of the British General Register Office based on occupation of father, from professionals (I) to unskilled workers (V). Source: Morris et al. (20). might have been expected, but equally clear is the increase in mortality with number of children in all social classes. The data depicted also support the contention that deaths due to causes associated with prenatal factors-congenital malformations—do not vary with family size or social class as do those associated with environmental factors, such as infections. Variations with age and parity of the mother of postneonatal and second-year mortality are shown in Figure 2. The persistence of increasing mortality with increasing family size throughout the first 2 years of life is obvious and *The data in these papers were subsequently brought together and issued as (28).e association between family size and infant mortality conies from the studies of Morris, Heady, Morrison, and their associates in a study of all the births that occurred in England and Wales during 1949 and 1950. They matched the information on the death certificates of all children dying under as the probability of picking one from a small family. In this situation, one would conclude that average size of family is 9.2 children. . . .